1. Field of the Invention
This invention relates to an image heating apparatus such as a heating and fixing apparatus mounted on a copier or a printer, or an apparatus for improving the surface state of an image, and particularly to an image heating apparatus for generating heat by the use of the principle of electromagnetic induction.
2. Related Art
For example, in image forming apparatuses such as printers, copiers and facsimile apparatuses adopting the image forming process of the electrophotographic type, the electrostatic recording type or the like, there are known and have been put into practical use heating apparatuses of various types based on various principles for heat-fixing an unfixed toner image of desired image information formed and borne on a recording material (such as a transferring material, printing paper, photosensitive paper or electrostatic recording paper) by a transferring process or a direct method in an image forming process portion as a fixed image.
Various types have also been proposed in the heating apparatuses of the electromagnetic. induction heating type as described above to which the present invention is directed.
For example, in Japanese Patent Application Laid-Open No. 10-31389, there is proposed an induction heating and fixing apparatus comprised of a fixing roller, a core passing through the interior of the fixing roller for forming a closed magnetic circuit, first and second coils wound on the core and series-connected together, and a power supply circuit for supplying an alternating current to the first and second coils, and having a control device for switching the circuit so as to supply the alternating current from the power supply circuit to both of the first and second coils for a predetermined time from the start of the temperature rise of the fixing roller, and to supply the alternating current onto to the first coil from after the lapse of the predetermined time, and designed to suppress the current flowing to the coils at the early stage of the temperature rise, and prevent the temperature rise speed from becoming low to the utmost.
Also, in Japanese Patent Application Laid-Open No. 2000-66543, there is proposed an induction heating and fixing apparatus comprised of a heating roller in which an induction heating coil comprising a lead wire wound on a winding frame is disposed, a movable core and two coils, wherein the movement of the core or a current supplied to the coils is limited in conformity with the temperature of the axial end portions of the heating roller, and a magnetic flux passing through the end portions is varied to thereby eliminate the unevenness of the temperature of the surface of the fixing roller in the axial direction thereof resulting from the fixing operation.
Also, in Japanese Patent Application Laid-Open No. 10-74009, there is disposed a fixing apparatus of the induction heating type in which magnetic flux intercepting means for intercepting part of a magnetic flux reaching a metal sleeve from an excitation coil is disposed between the metal sleeve and the excitation coil, and the position of the magnetic flux intercepting means is changed by displacing means in conformity with a sheet passing range in the metal sleeve, whereby the heat distribution of the metal sleeve rising in temperature is made controllable irrespective of the size of a recording material passed.
The image heating apparatuses as described above are for fixing a toner image on a recording material, and highly accurate temperature control is required of them so that the fixativeness of the toner may become optimum. Therefore, it is necessary to effect temperature control so that the temperature of the metal sleeve contacting with the toner image may not greatly deviate from a target temperature.
In the three publications mentioned above, it is described to switch the supply of electric power to the two coils or move the core as required.
Such an operation, however, results in a fluctuation in the input impedance of the coils.
When the input impedance changes, the electric power supplied to the coils fluctuates and therefore, even if for example, the supply of electric power to the coils is controlled so that the temperature detected by a temperature sensor for detecting the temperature of the metal sleeve may maintain a set temperature, desired electric power supply is not obtained and therefore, the temperature rise of the metal sleeve is delayed and the ripple of the temperature of the metal sleeve becomes great.
The fluctuation of the input impedance also depends on the temperature of the heating apparatus. That is, the fluctuation of the electric power supplied to the coils also depends on the temperature of the heating apparatus.
As described above, the input impedance is fluctuated by a change in the farm or temperature of the heating apparatus and the electric power supplied to the coils is not stable, and the ripple of the temperature of the metal sleeve becomes great. Consequently, the fixativeness of the toner is affected.
The present invention has been made in view of the above-noted problem and an object thereof is to provide an induction heating apparatus in which the ripple of the temperature of a heating member can be made small.
Another object of the present invention is to provide an induction heating apparatus in which optimum electric power supply can be effected even if the input impedance of the heating apparatus changes.
Still another object of the present invention is to provide an induction heating apparatus comprising:
heating means for heating an image on a recording material, the heating means including a heating member and an excitation coil for generating a magnetic flux to induce an eddy current in the heating member;
an input impedance of the heating means being variable;
a temperature detecting element for detecting a temperature of the heating member; and
control means for controlling a frequency of electrical supply to the excitation coil so that the temperature detected by the temperature detecting element is maintained at a set temperature;
wherein the control means controls the frequency in accordance with the temperature detected by the temperature detecting element and information regarding the input impedance of the heating means.
Further objects of the present invention will become apparent from the following detailed description when read with reference to the accompanying drawings.